This application claims benefit of priority to Japanese Application No. JP 2000-248624 filed Aug. 18, 2000, the entire content of which is incorporated by reference herein.
1. Field of the Invention
The present invention relates to a method of mounting whereby a BGA (ball grid array) such as an FBGA (fine ball grid array) is soldered onto a printed circuit board.
2. Description of the Related Art
A prior art example of a method of mounting etc. whereby a BGA is soldered onto a printed circuit board is described using FIG. 1 to FIG. 5.
First of all, the method of mounting the BGA is described in FIG. 1 and FIG. 2.
FIG. 1 is a view showing the condition prior to mounting the BGA on the printed circuit board. The general structure of a BGA is that BGA-side pads 5 constituting electrodes are arranged in grid fashion on the underside of a BGA body 4 and solder balls 6 are connected to the surfaces of the BGA-side pads 5 (these electrodes are referred to as bump electrodes).
In order to mount a BGA on a printed circuit board, solder paste 3 is applied beforehand onto printed circuit board-side pads 2 constituting electrodes arranged in the same arrangement as the BGA-side pads 5 on the printed circuit board 7, and mounting is effected such that the solder balls 6 are superimposed on the printed circuit board-side pads 2. In FIG. 1, numeral 1 are through-hole upper lands, numeral 8 are through-hole internal portions, and numeral 9 are through-hole bottom lands. These 1, 8 and 9 are connected to the printed circuit board-sides pads 2, and then the printed circuit board-sides pads 2 are respectively connected to every circuit (not shown) via through-holes.
After mounting, the BGA and printed circuit board are heated in a reflow device or the like, not shown, so that soldering is effected by melting the solder 6 and solder paste 3.
FIG. 2 is a view showing the condition after the BGA has been mounted on the printed circuit board.
The molten solder 6 and solder paste 3 fuse to a single mass which wets and spreads out over printed circuit board-side pads 2, thereby forming a solder fillet (condition in which the BGA-side pads 5 and printed circuit board-side pads 2 are connected through bump electrodes) between the BGA-side pads 5 and printed circuit board-side pads 2, which are thus soldered together.
In this process, if the amount of solder paste 3 is too large, the solder spreads out, effecting joining between the printed circuit board-side pads 2 i.e. solder bridges 12 are produced. Also, if the amount of solder paste 3 supplied is too little or BGA solder balls are missing, soldering is not effected between the BGA-side pads 5 and the printed circuit board-side pads 2, giving rise to solder gaps 11 (condition in which the amount of solder is too little).
In the case of the solder joints of ordinary mounted components, not shown, the presence of solder gaps or solder bridges can be ascertained by visual observation of the solder fillets formed between the leads or electrodes and the pads of the printed circuit board of the mounted components, since the mounted components are on the outside. Also, the condition of the solder fillets can be automatically inspected using a soldering appearance inspection device which makes use of a camera and/or laser to make this visible to the eye.
FIG. 3 is a view of a transmission image obtained using X-rays of solder joints of a BGA and printed circuit board pads. The lead contained in solder 6 does not easily transmit X-rays and so shows up as dark patches in the transmission image. The transmission image of normal solder joints appears circular, but in the condition where there is a solder bridge 12, an oval-shaped transmission image is produced between adjacent pads.
Also, in the case of a solder gap 11, since the amount of solder is small, a transmission image of a smaller circular shape than the transmission image of a normal soldered joint is produced. Thus, since the BGA solder fillets are sandwiched between the printed circuit board 7 and BGA body 4, they are not visible to the naked eye and it is necessary to observe their transmission images using X-rays etc. Thus the inspection step of a printed circuit board on which a BGA was mounted was performed divided into separate steps for the BGA section and the other mounted components.
FIG. 4 is a view showing a condition in which the BGA is removed using a repair device.
In the ordinary method of repairing soldering defects of mounted components, not shown, in the case of solder bridges, the location thereof is heated using a soldering iron and the solder is removed by melting. Also in the case of solder gaps, the location of the gap is heated with a soldering iron and repair is effected by supplying solder thereto.
However, in order to repair a BGA, an expensive special-purpose repair device is required. When solder bridges 12 of adjacent printed circuit board-side pads 2 occur or solder gaps 11 of BGA-side pads 5 and printed circuit board-side pads 2 occur, even though these occur at only a single location of a BGA having some hundreds of soldering joint locations, the entire BGA must be removed and a new BGA re-mounted.
As shown in FIG. 4, regarding the method of removing the BGA, removal from the printed circuit board is effected by heating printed circuit board 7 from below using a heater 16 and raising the BGA body 4 by suction using a suction nozzle 15, after melting all of the solder of the BGA by using a heated air current nozzle 14 to heat the BGA by blowing onto it, from above the BGA, a current of hot air from a heated air current generator, not shown. Since the solder of the BGA-side pads 5 of this BGA that has been removed has become nonuniform, it cannot be reused. Solder remaining behind on the printed circuit board-side pads 2 is flattened using a solder suctional removal device, not shown.
Next, the method of remounting the BGA is that a new BGA is prepared, and positional location is performed by suctional attachment of the BGA using suction nozzle 15, after which, in the same way as in the case of removal from the printed circuit board, heating is effected from below printed circuit board 7 using a heater 16 and heating of the upper side of the BGA is effected by blowing onto the BGA by a heated air current nozzle 14 a current of hot air from a heated air current generator, not shown, thereby melting all of the solder 6 of the BGA to effect soldering.
FIG. 5 is a view showing the condition in which heat radiating fins 17 of the BGA are mounted.
BGA packages are often employed in computer CPUs or MPUs and the amount of heat generated on passage of current is increasing as the speeds of the CPUs or MPUs are increased.
However, with the conventional method of BGA mounting, a soldering portion gap is present between the BGA body 4 and printed circuit board 7, so heat removal by transfer of heat to the printed circuit board was not possible. Heat radiating fins 17 or the like were therefore necessary to allow the heat to escape. Since the heat radiating fins 17 were mounted on the top of the BGA body 4, they presented an obstacle to reducing the thickness of the product.
As described above, in inspecting solder joints of a BGA using the conventional BGA mounting method, the inspection had to be divided into two inspection processes, namely, inspection of the soldered joints of the ordinary mounted components and inspection of the BGA soldered joints. For inspection of the BGA solder joints, an expensive X-ray apparatus was necessary. This therefore increased the inspection costs due to increase in the number of inspection steps and tended to prolong the lead time for manufacturing printed circuit boards. In addition, there was a problem that an expensive repair device was necessary for repairing defects in BGA soldered joints. Also, since heat is generated on passage of current through the BGA, heat radiating fins and/or a heat removal fan had to be provided.
Accordingly, one object of the present invention is to provide a novel BGA mounting method wherein the foregoing problems can be eliminated, wherein checking of the quality of BGA soldered joints is possible by a method of inspecting the external appearance of the soldering of the solder joints as is done in the case of ordinary mounted components, an X-ray apparatus and repair device are not required, and mounting can be realized that copes with demands for reduction in weight, thickness, length and size and reduction in lead time.
In order to achieve the above object, the present invention is constructed as follows. Specifically, it consists in a BGA mounting method in which a BGA is soldered onto a printed circuit board wherein solder paste is applied beforehand onto at least one of lands of the printed circuit board which are in the position of through-holes of the printed circuit board and solder balls of the BGA which are connected with pads of the BGA; attachment is effected such that the through-holes of the printed circuit board and the solder balls of the BGA are superimposed; by heating the BGA and the printed circuit board, the solder balls and the solder paste are melted and flow into the through-holes, being led into the through-hole bottom face lands which they wet and where they spread out; and the pads of the BGA are stuck securely together with the lands of the printed circuit board which are in the positions of the through-holes of said printed circuit board and effect solder joints therewith, and solder fillets are formed on said through-hole bottom face lands.
With the present invention, the quality of the solder joints of the BGA can be ascertained by observing the solder fillets formed on the BGA through-hole bottom face lands.
It therefore also becomes possible to apply an automatic soldering inspection device such as is employed for solder fillet inspection of ordinary mounted components, thereby enabling the inspection time to be greatly reduced.
Also, since even if excess solder paste is supplied, the excess solder melted by heating is drawn through the through-holes by the capillary effect (capillary action, or capillarity) and wetting effect of the through-holes and collects on the through-hole bottom face lands, so no solder bridges are generated on the BGA pad side.
Furthermore, even if soldering gaps are generated at locations where BGA solder balls are absent, these can be repaired by supplying solder by applying heat from the through-hole bottom face lands.
Also according to the present invention, if the through-hole volume is VTH and the solder volume of the hemispheres on the through-hole bottom face lands is VLAND, the total amount of solder VSOLDER obtained by adding the solder and the solder paste required for solder joining of the BGA pads and the lands of the printed circuit board which are in the positions of the through-holes of the printed circuit board is in the range VTH less than VSOLDER less than VTH+VLAND.
As a result of evaluation tests of this BGA mounting method it has been found that if the amount of solder within the through-holes is small, the surface tension of the solder acts in the direction of the interior of the through-holes, causing the solder to separate from the BGA pads, resulting in soldering gaps.
Also, if the amount of solder is too great, the solder assumes a condition in which it swells up on the through-hole bottom face lands, making contacts with solder on adjacent through-hole bottom face lands, producing solder bridges. From results of these evaluation tests, the amount of solder necessary for the BGA mounting can be accurately calculated in accordance with the volume of the through-holes.
According to the present invention, the appropriate amount of solder can be calculated in a straightforward fashion for any combination of sizes of the BGA solder balls and the through-holes, so stable BGA mounting can be achieved with no solder defects.
Also design standards can be established for the pad sizes and the through-hole diameters etc. of the printed circuit boards to which the present method of BGA mounting is applied.
Furthermore, according to the present invention, a ground layer pattern may be provided in a position of the printed circuit board corresponding to the position of the silicon chip of the BGA, and the BGA body and the ground layer pattern securely stuck together.
According to the present invention, efficient radiation of the heat generated by passage of current through the BGA can be achieved. Also, heat radiating fins and/or a heat removal fan are unnecessary, making it possible to reduce the thickness of the product.